1. Field of the Invention
This invention relates to the manufacture of molded articles by reaction injection molding of a urethane composition. It is more particularly concerned with RIM formulations particularly characterized as having improved moldability.
2. Description of the Prior Art.
A recent advance in the area of urethane technology has been the use of reaction injection molding, also known as RIM. This method of molding uses high pressure, multi-stream, high velocity impingement whereby essentially instantaneously mixing is effected. In RIM technology the mixed urethane foam ingredients are injected into the mold cavity through a runner and gate at low pressures, usually about 20 psi, where they react very quickly to produce the molded article. Large parts can thereby be produced at low mold pressures--generally less than 60 psi.
Such elastomers normally comprise the reaction product of an aromatic polyisocyanate, a polyol and a chain-extending agent.
One type of polyol employed is a blocked polypropylene glycol with an ethylene oxide cap to form a low flex modulus elastomer. Said polyol usually comprises a glycol initiator having an internal blocked polypropylene oxide segment with a polyethylene oxide cap. See, for example, U.S. Pat. Nos. 3,535,307 and 2,125,505 and R. M. Gerkin and F. E. Critchfield, "Factors Affecting High and Low Temperature Performance in Liquid Reaction Molding Urethane Elastomers", SAE paper presented at Automative Engineering Meeting in Toronto, October, 1974.
However, while elastomers from polyols of the type just described have significant utility in the RIM area, they do possess some drawbacks, particularly with respect to molding properties. For example, in using such polyols it has been noted that a buildup of urethane residue occurs on the mold surface. In addition, there is noted some cracking of the skin if the part is bent during molding. Lastly, the part so molded in many instances demonstrates shrink marks.
It would be a particular improvement in the art if the above type of polyols could be somehow improved so as to rectify the moldability problems and thus produce suitable flexible elastomers for applications such as automobile fascia and other parts.
It therefore becomes an object of the invention to minimize the just discussed problems with moldability or eliminate same by resorting to an improved reaction injection molding polyurethane formulation containing a specific class of high molecular weight polyols designed specifically to aid in enhancing demolding properties of elastomers therefrom.